Photomultiplier



May 13, 1969 BETOULE ET AL 3,444,414

PHOTOMULT IPLIER Filed Jan. 4, 1967 INVEN'IORJ MICHEL BETOULE REN PETITJEAN-CLAUDE REBUFFIE AGENT nited States Int. Cl. lj 39/12 U.S. Cl. 313964 Claims ABSIRACT OF THE DISCLOSRE A high-eurent fast-responsemultiplier employing an anode having a small aperture therein pos1t1onedbetween the last two dynodes and in front of the last dynode.

The present invention relates to a photomultiplier the exit stage ofwhich consists of three electrodes for the emission, the focussing, themultiplication and the collection et a flux of electrons, namely thenext to the last dynode, the last dynode and the anode, the anode bemgpositioned between the next to the last dynode and the last dynode andjust in front of the latter dynode. The invention particularly concernsa high current photomultiplier With a fast response.

In a photomultiplier a photocathode liberates a flux of electrons uponcapture of a flux of photons. The flux of electrons is multiplied bysuccessive secondary emission of a plurality of electrodes, calleddynodes, each dynode thereby focussing the flux of electrons onto thenext dynode. In the exit stage of the photomultiplifier the electronsoriginating from the next to the last dynode are focussed onto the lastdynode, which like the preceding dynodes gives rise to secondaryemission. The electrons originating from the last dynode are focussedonto and collected by an anode, from which the output signal can betaken. There are known photomultipliers wherein the anode is a gridpositioned between the next to the last dynode and the last dynode andjust in front of the latter, so as to obtain a strong electric field anda maximum active surface.

These multipliers have some disadvantages, which are particularlynoticeable in the case of high currents With fast variations. As amatter of fact, a part of the electrons originating from the next to thelast dynode are directly captured by the grid without being subjected tomultiplication by the last dynode. On the other hand, a part of theelectrons originating from the last dynode and being attracted towardthe grid, which has a higher potential than this dynode, pass throughthe grid and this introduces a perturbation of the output signal.

It is an object of the invention to provide a photomultiplier, whichbeing free from said disadvantages has improved characteristicsconcerning the fastness of the response and its degree of linearity.

In accordance With the invention, in a photomultiplier, the exit stageof which consists of three electrodes for the emission, the focussing,the multiplication and the collection of a flux of electrons, namely thenext to the last dynode, the last dynode and the anode, the anode beingpositioned between the next to the last dynode and the last dynode andjust in front of the latter dynode, the anode has a single opening of asmall cross-section With respect to the next to the last dynode.

Electron tubes for the generation or the amplification of oscillationsof very high frequencies are known, in which anodes are used having asingle opening in order to reduce the time of transit of the electrons.These tubes atcnt O show a slight resemblance to the exit stage of thephotomultiplier according to the invention. Like this exit stage theyinclude an electrode for the production of a flux of electrons, anelectrode for the multiplication of this flux by means of secondaryemission and an anode for the collection of electrons. However, theyessentially diier from the exit stage of the photomultiplier in thatthey include at least a fourth electrode for the focussing of theelectrons, without which they would not serve their purpose. Besidesthey have no preceding electrode structure. As a matter of fact thesetubes belong to a diflerent art and the problems arising inphotomultipliers are by no means comparable With the ones in said tubes.The differences originate mainly from the fact that photomultipliers aremeasuring apparatus, in which there must be conserved a linearity in thea prior unknown pulse heights. Moreover, the electron beams in thephotomultipliers have large cross-sections, s0 their focussing problemsare of a very particular nature.

Nevertheless it has been observed that the flux of electrons originatingfrom the next to the last dynode was focussed in a cross-section of anoticeable smallness by a structure of the three electrodes of the exitstage being convenient in every respect. This permitted the use of ananode having a single opening of small dimensions With respect to thesurfaces facing it.

In this structure, provided with an anode having a single opening ofsmall dimensions, almost the total flux of electrons originating fromthe next to the last dynode passes through the opening in the anode. Onthe other hand the smallness of the dimensions of this opening preventsalmost completely the passage of electrons through the anode in theopposite direction.

Preferably the anode is substantially equidistant to the emissivesurface of the last dynode in order that the electrons originating fromthe last dynode substantially have the same time of flight.

Preferably the emissive surface of the last dynode presents a convexityoriented to the opening in the anode. For instance it may have the formof a dihedral angle. This convexity provides an additional reduction ofthe number of electrons originating from the last dynode that passthrough the opening in the anode.

The invention will be described with reference to the accompanyingdrawing.

The drawing shows a cross-section of the tube of a photomultiplier. Thistube has the form of a cylinder of revolution its axis beingperpendicular to the plane of the figure. The electrodes in the tubehave the form of cylindrical surfaces with generatrices parallel to thisaxis. The photocathode 10 consists of an opaque silver foil covered withcaesium antimony alloy.

The photocathode =10 emits a flux of electrons upon capture of a flux ofphotons indicated in a general manner by means of the arro=ws 1. Theelectrode 2 has a positive potential of some hundreds of volts Withrespect to the cathode 10 in order to deflect the electrons emitted bythe cathode 10 in the direction of the first dynode 11, which has apositive potential with respect to the cathode 10. The impact of theseprimary electrons on the dynode 11 gives rise to the emission ofs0-called secondary electrons, the number of which is usually three tofive times the number of the primary electrons. These secondaryelectrons are in their turn attracted by the dynode 12 and give rise tosecondary emission from the dynode 12. The process is repeated at thedynodes 13, 14 and &15. The dynodes consist of fols of a silvermagnesiumalloy. In the space between the dynodes are shown the metal posts 21, 22and 23. These serve t0 improve the trajectories of the electrons bymeans of an electrostatic action, which permits a better repartition ofthe action On the dynodes. Moreover, they reduce the time of transit ofthe electrons and the influence of the space charge, which limits theflux of electrons. The posts 21 and 22 have the potentials of thedynodes 14 and 15. The post 23 has a potential intermediate between thepotentials of the dynodes 15 and 16. The last dynode 16 presents anemissive surface which has the form of a dihedral angle. The anodeconsists of two parts: a post 17a parallel to the generatrices of thedynodes and opposite to one face of said dihedral angle anda part With afiat surface 17b parallel and just opposite to the other face of saiddihedrai angle. The space between the two parts of the anode (which havethe same potential of the order of 3 kV. with respect to the cathode)forms an opening 18 the length of which is parallel to the generatricesof the dynodes. A secondary electron emitted by the dynode :15 passesthrough the opening 18, impacts on the dynode 16 and liberates severalelectrons, which are collected by the anode 17. The potentials of thedifierent elements are adjusted so as to obtain a focussing in the planeof the opening 18. Conductors 34 and 35 support getters. The references40 to 46 are bars, which support the structure within a glass cylinder.Moreover, these bars serve as conductors.

It is possible to conceive several modifications of the electronmultiplier, wvhich remain within the scope of fe invention, particularlyconcerning the number of dynodes.

What is claimed is:

:1. A photomultiplier comprising a source of primary electrons, aplurality of dynodes in succession and an anode for collecting electronsliberated by the last of the dynodes, said anode being positionedbetween the last dynode and the next preceding dynode, said anode havinga single aperture therein the crosssection of which is small relative tothe next preceding dynode, said last dynode having a convex emissivesurface orientcd toward said anode aperture.

2. A photomultipler according to claim 1, character ized in that allpoints on the anode surface are substantially equidistant from theemissive surface of the last dynode.

3. A photomultper according to claim 2, characterized in that theemissive surface of the last dynode has the form of a dihedra] angle.

4. A photomultipfier according to claim 3, character ized in that theanode comprises a post parallel and just opposite to one face of saiddihedral angle, and a part having a fiat face parallel and just oppositeto the other face of said dihedral angle.

References Cited UNITED STATES PATENTS 2,198,233 4/1940 Snyder 3132,231,693 2/1941 Snyder 313105 2,285,126 6/1942 Rajchman et a]. 3131053,260,878 7/1966 L6g0ux 313104 IAMES W. LAWRENCE, Primary Examiner.

DAVID OREILLY, Assistant Examiner.

U.S. Cl. X.R.

